Patent GB-822552-A (GB)
Title: Improvements in and relating to indexing in colour television receivers
Inventor: OXENHAM JOHN KENNETH
IPC: H04N9/24
Filed: 1956-02-15
Abstract: 822,552. Colour television. SYLVANIATHORN COLOUR TELEVISION LABORATORIES Ltd. Jan. 31, 1957 [Feb. 15, 1956], No. 4780/56. Class 40 (3). [Also in Group XL (c)] Relates to a colour television system, Fig. 1, utilizing a cathode-ray tube 16 having a screen formed of a repeating sequence of red, green and blue phosphor stripes, Fig. 2, and incorporating or co-operating with means, such as photo-electric cell 25, to derive a signal each time the scanning electron beam traverses an indexing region of the screen, e.g. a stripe of a selected colour, and which is employed to control the application of colour signals to the modulating electrode of the tube in step with the scanning. In accordance with the invention, the indexing signal is amplified in an amplifier 26 which is arranged to apply an output to the grid 15 of the cathode-ray tube so as to complete a loop circuit including the cathode-ray beam, the gain of the loop circuit being made such that the circuit changes from a non- oscillatory to an oscillatory state each time the beam passes on to an indexing region. The indexing information is thus derived by a circuit operating on an " on-off " basis and is not affected by non-linearity in the cathode-ray tube which otherwise would cause the colour signal information to modify the amplitude and phase of the indexing signal. The gain of amplifier 26 is chosen in relation to the indexing ratio (i.e. the ratio of the signal obtained in response to a beam on and off the indexing region) and the Specification discusses this feature as applied both to a system employing a photo-electric cell, Fig. 1, and one in which the indexing signal is derived by secondary emission stripes incorporated in the tube screen, Figs. 3 and 4 (not shown). In this latter system a high effective index ratio is obtained by deriving the output signal from the screen rather than from the secondary emission collector electrode. Reference is made to systems in which the loop is non-oscillatory when the beam is on an indexing region and becomes oscillatory when it moves off a region. Specification 753,451 is referred to. It is also possible, by selecting the secondary emission co-efficients of the indexing stripes and the remainder of the screen to be such that one is greater and the other less than unity, to arrange the indexing signal to change in phase by 180 degrees as the beam passes over the indexing regions. The loop is then arranged to be oscillatory with a signal of one phase and thus becomes non- oscillatory for a signal of the other phase. As illustrated in Fig. 1, the indexing signal is applied to a detector 28 to derive a carrier w1 on which the colour information is modulated in a stage 10. The bandwidth of the loop circuit is made sufficient to include a carrier frequency #0 and at least one side-band due to the indexing information. The output from the amplifier is applied to the tube grid to complete the loopcircuit by means of an adder 13 which also receives a luminance signal. A limiter 27 is included in the loop in order to limit the amplitude of the oscillations to a value small enough to avoid a shift in the luminance values due to the rectifying action of the cathode-ray tube.